1. Field of the invention
This invention relates to a photocoupler apparatus having an output MOSFET at the secondary side in general, and more particularly, to the photocoupler apparatus which is capable of limiting the output current flowing through the conducting electrodes of the output MOSFET below a certain value.
2. Description of the prior art
In FIG. 1, the circuit structure of a photocoupler apparatus is shown according to one prior art of this invention. This apparatus is described in "Optically Coupled Power MOS FET Technology: A Monolithic Replacement For Electrome chemical Relays", Electro '83, Session Record 7/3, IEEE 1983.
As shown in FIG. 1, this photocoupler apparatus has a light emitting diode 1 at its primary side for emitting light when receiving input signals through terminals 6 and 6'. In the secondary side of this apparatus, there are provided a first photodiode array 2a, a discharging MOSFET 10b, a second photodiode array 10a, a first and a second output MOSFETs 5 and 5', a first and a second parasitic diodes 5a and 5a', a first and a second current detecting resistors 4 and 4', and a first and a second bipolar transistors 3a and 3a'. In this apparatus, first photodiode array 2a generates electromotive force when receiving a light signal from light emitting diode 1. On the other hand, the second photodiode array 10a drives discharging MOSFET 10b by receiving the light signal from light emitting diode 1. In addition, bipolar transistors 3a and 3a' operate as current limiting circuits.
In said apparatus, current detecting resistors 4 and 4' are connected in series with the conducting electrodes of first and second output MOSFETs 5 and 5'. The base-emittor junctions of bipolar transistors 3a and 3a' are biased by the voltage difference developed across resistor 4 or 4', thus controlling the gate-source voltages of the first and the second output MOSFETs. As a result, the current which flows through the conducting electrodes of MOSFETs 5 and 5' are limited below a certain value.
So, the output current flowing across terminals 7 and 7' are limited depending on the resistance values of current detecting resistors 4 and 4' and the base-emittor voltages VBE (that is, VF of base-emittor junction) of bipolar transistors 3a and 3a'. In usual, the VBE of bipolar transistors 3a and 3a' made of Si is about 0.5 V. Therefore, a voltage greater than 0.5 V should be developed across each resistor 4 and 4' in order to limit the output current.
Due to the above mentioned reason, the resistance of current detecting resistors 4 and 4' can't be reduced below a certain value which depends on the current value to limit. This fact increases not only on-resistance Ron of a final product (on-resistance of output MOSFETs 5 and 5' +values of resistors 4 and 4'), but its heat loss. As a result, the prior art photocoupler apparatus is disadvantageous in that the current capacity of this apparatus can't be made so large. In other words, the apparatus can't accept a large current.